1. Field of the Invention
This invention relates to the drilling of wells, and more particularly concerns an improved method for drilling a well through a formation containing unconsolidated dolomite.
2. Description of the Prior Art
Drilling a well through an incompetent subterranean formation has presented many problems. Conventionally, aqueous-base drilling fluids containing water, clay and various additives are circulated through the bore hole during the drilling operation to carry drill cuttings from the bore hole to the surface. These clay-containing drilling fluids form a mud cake on the bore hole walls which reduces the sloughing of the incompetent formation as long as the fluid pressure in the bore hole due to the standing column of drilling fluid, exceeds the pressure of the connate fluid in the incompetent formation. Therefore, drilling through typical incompetent formations per se is not particularly a problem. However, it is commonplace to encounter a lost circulation zone, either in the incompetent formation itself or in an underlying strata, or to lose the column of drilling fluid due to the cumulative effect of individually manageable fluid losses to a plurality of formations. When the lost circulation results in a fluid pressure in the bore hole less than the fluid pressure in the adjacent incompetent formation, the formation will slough into the bore hole. A stuck drill string often results from this sloughing and may lead to abandonment of the drill string and the well.
The incompetent formations normally encountered in drilling oil and gas wells are generally formations which contain quartz sand and/or clay. Accordingly, the prior art methods of consolidating these formations have been directed to injecting one or more chemical solutions to react with the quartz sand or clay to form a consolidated sheath about the well bore. Depending upon whether or not it is desirable to permanently seal the formation from the well bore, the chemical solutions can be selected such that the resulting sheath is either permeable, such as disclosed in U.S. Pat. No. 3,175,611 to Hower, or substantially impermeable, such as disclosed in U.S. Pat. No. b 2,207,759 to Reimers.
Dolomite formations encountered in drilling oil and gas wells are generally competent unless the formation also contains a significant amount of quartz sand or clay. Accordingly, dolomite formations which are substantially free of quartz sand and clay have not required consolidation. However, subterranean formations containing one or more zones of unconsolidated dolomite have been encountered in drilling core holes for minerals exploration, and in drilling wells to explore geothermal prospects. The incompetent dolomite is believed to result when a zone of dolomite crystals dispersed in a calcite matrix is subjected to ground water leaching, such as hydrothermal leaching by a geothermal fluid. The ground water selectively leaches the more soluble calcite matrix materials to leave the unconsolidated, finely divided dolomite crystals. These crystals are relatively free flowing and are prone to sloughing into a void space, such as a bore hole.
Previous attempts to drill through these unconsolidated dolomite formations have been unsuccessful. Numerous core holes and at least one well drilled to explore a geothermal prospect have been abandoned due to this problem. Prior art consolidation methods have been found to be inadequate for the consolidation of the incompetent dolomite formations. Therefore, a need exists for a method for drilling through incompetent dolomite formations.
Accordingly, a principal object of this invention is to provide a method for drilling a well through incompetent dolomite formations.
Another object of this invention is to provide a method for drilling through a hydrothermally-leached formation containing incompetent dolomite.
Yet another object of the invention is to provide a method for consolidating an incompetent dolomite formation during the drilling operation sufficiently to prevent subsequent sloughing of the formation into the bore hole during the drilling operation.
A further object of the invention is to provide an improved drilling method by which a relatively thick incompetent zone of dolomite can be penetrated while avoiding sloughing of the dolomite into the bore hole and while reducing fluid loss to the incompetent formation.
Further objects, advantages and features of the invention will become apparent to those skilled in the art from the following description taken in conjunction with the accompanying drawings.